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The AAPG/Datapages Combined Publications Database
Houston Geological Society Bulletin
Abstract
Abstract: Exploitation of Thin Basin-floor Fan Sandstones, Navarro Formation (Upper Cretaceous), South Texas
By
ChevronTexaco
North America Upstream
During thirty years of industry drilling
in the Paleocene Lobo Trend of South
Texas, thin sandstones of the Upper
Cretaceous Navarro Formation have been
regarded as a high-risk secondary objective
that occasionally pays the cost of drilling an
additional 1,000 feet to test it. Several
recent completions have yielded impressive
sustained flow rates in excess of 1 million cubic feet of gas per
day (MMCFGPD) per vertical
foot of reservoir, therefore justifying
an effort to better understand its occurrence.
The Navarro reservoir in southern Webb and northern Zapata Counties is a thin sporadically-occurring sand encased in deepwater shales that occurs basinward of the Cretaceous shelf margin. It is interpreted as a basin-floor fan based on log character and paleontologic bathymetric analysis. The sand averages 10 feet in thickness and cannot be resolved seismically as a discrete event, however, areas favorable for sand accumulation can be predicted using seismic attributes derived from 3-D volumes.
Where the sand thickness exceeds 15 feet a good correlation exists
with the amplitude value of the seismic peak associated with the
sand top. However, in most areas the sand is thinner and accommodation
space in subtle intrabasinal
depressions can be inferred by 3-D isochron
mapping. Most areas that have Navarro sand
correlate with isochron thicks; however, not
all isochron thicks have sand, most likely
because sediment supply was less than the
available accommodation space. These
attributes should be applicable in other areas
in which seismic resolution
of a sand body is difficult.
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